The VIDYANKER team has thoughtfully prepared the NCERT Solutions for Class 11 Biology Chapter 10, "Cell Cycle and Cell Division Biology" These solutions are designed to help you tackle the NCERT textbook questions with ease. We recommend going through the chapter's theory before diving into the solutions for a deeper understanding. Feel free to share these NCERT Solutions for Class 11 Biology with others learning is always better when shared!
NCERT TEXTBOOK QUESTIONS SOLVED
1. What is the average cell cycle span for a mammalian cell?
Ans. 24 hours.
2. Distinguish cytokinesis from karyokinesis.
Ans.
| Cytokinesis | Karyokinesis |
|---|---|
| Cytokinesis refers to the separation of the cytoplasm that happens at the termination of the cell cycle, where two distinct daughter cells are formed. It starts during the latter part of the mitotic phase. | Karyokinesis refers to the division of the cell’s nucleus during mitosis or meiosis, resulting in the separation of the genetic material into two sets, one for each daughter cell. |
3. Describe the events taking place during interphase.
Ans. The interphase, although referred to as the resting phase, is the period when the cell is getting ready for division by growing and replicating its DNA. The interphase is-segmented into three additional phases:
→ G1 phase - It is the phase when the cell grows and gets its DNA ready for replication. During this phase, the cell is metabolically active.
→ S phase - It is the phase in which DNA synthesis takes place. During this phase, the DNA content (per cell) doubles but the number of chromosomes does not change.
→ G2 phase - During this phase, the cell continues to increase in size and gets ready to divide. Proteins and RNA needed for mitosis are synthesised during this phase.
4. What is G0(quiescent phase) of cell cycle?
Ans. G0 phase is the inactivation phase of the cell cycle owing to the lack of availability of mitogens and energy rich substances. Cells of this stage are metabolically active but no longer divide i.e., do not grow or differentiate unless summoned to do so according to the organism's need. E.g., Nerve and heart cells of chordates exist in permanent G0 phase.
5. Why is mitosis called equational division?
Ans. Mitosis is a cell division in which chromosomes are duplicated and become equally divided in two daughter nuclei such that the daughter cells end up having the same number and type of chromosomes as found in parent cell. Thus mitosis is referred to as equational division.
6. Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
(iv) Crossing over between homologous chromosomes takes place.
Ans. (i) Metaphase
(ii) Anaphase
(iii) Zygotene of meiosis I
(iv) Pachytene of meiosis I
7. Describe the following:
(a) synapsis (b) bivalent (c) chiasmata
Draw a diagram to illustrate your answer.
Ans. (i) At zygotene of prophase I stage homologous chromosomes begin to pair with each other and this association is referred to as synapsis. Electron micrographs of this stage reveal that chromosome synapsis is associated with the development of complex structure known as synaptonemal complex.
(ii) The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad i.e., 4 chromatids or a pair of chromosomes.
(iii) Chiasmata is the location where two non-sister chromatids of homologous chromosomes have crossed over. It is the location of cross-over. It is developed during the diplotene stage of prophase I of meiosis.
8. How does cytokinesis in plant cells differ from that in animal cells?
Ans.
| Plant Cytokinesis | Animal Cytokinesis |
|---|---|
| The division of the cytoplasm takes place by cell plate formation. | The division of the cytoplasm takes place by cleavage. |
| Cell plate formation starts at the centre of the cell and grows outward, toward the lateral walls. | Cleavage starts at the periphery and then moves inward, dividing the cell into two parts. |
9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Ans. In the development of male gametes (sperms) in humans, the four daughter cells produced during meiosis are of equal size. In the development of female gametes (ova), three smaller polar bodies are formed, leaving one large ovum, and thus unequal daughter cells.
10. Distinguish anaphase of mitosis from anaphase I of meiosis.
Ans.
- Anaphase of mitosis: Anaphase is the stage during which the centromere splits and the chromatids separate. The Chromatids move to opposite poles.
- Anaphase I of meiosis: The homologous chromosomes separate, while sisterchromatids remain associated at their centromeres
11. List the main differences between mitosis and meiosis.
Ans.
| Mitosis | Meiosis |
|---|---|
| This type of division takes place in somatic cells | This type of division takes place in gametic cells |
| Two daughter cells are formed | Four daughter cells are formed |
| Number of chromosomes remains diploid in daughter cells | Number of chromosomes becomes haploid in daughter cells are formed |
| Mitosis is necessary for growth and repair | Meiosis is necessary for sexual reproduction |
| Crossing over does not take place | Crossing over takes place |
12. What is the significance of meiosis?
And. Meiosis is necessary for sexual reproduction because it halves the number of chromosomes, making sure that offspring inherit the proper amount of chromosomes. It ensures genetic variation by crossing over and independent assortment, which is crucial for evolution.
13. Discuss with your teacher about
(i) haploid insects and lower plants where cell-division occurs, and
(ii) some haploid cells in higher plants where cell-division does not occur.
Ans. (i) In some haploid insects and lower plants, cell division is actively occurring in their life cycles. For instance, in honeybees, the male bees known as drones are haploid and are produced from unfertilized eggs by a process called parthenogenesis. In these haploid drones, mitotic cell division happens for growth and maintenance of body tissues, though meiosis is not present because they do not have to produce gametes through meiosis. Likewise, in lower plants such as algae, bryophytes, and certain fungi, the bulk of the plant body is haploid, and these plants demonstrate mitotic cell division during vegetative growth and reproductive structure development. These divisions assist in creating specialized organs such as antheridia and archegonia that produce gametes, all still in the haploid state.
(ii) In higher plants, the body is diploid, and the haploid stage is embodied in the gametophytic generation, which is brief and specialized. Some of the haploid cells in higher plants do not divide further at all. For example, the egg cell within the embryo sac is haploid and produced subsequent to meiosis, but it doesn't divide again unless fertilization occurs. Likewise, male gametes developed from pollen grains are haploid and likewise do not divide. Other cells like synergids and antipodals that belong to the female gametophyte are functionally active but do not divide. Such cells carry out their specific function in reproduction without dividing.
14. Can there be mitosis without DNA replication in S phase?
Ans. No, there can be no mitosis without the replication of DNA in S phase since extra DNAs are needed for the synthesis of new cells. Duplication of DNA is necessary as it keeps the chromosome number constant in the daughter cells and thus Mitosis is an equational division. Hence, the duplication of DNA is a necessary process and without this process, no mitosis can occur.
15. Can there be DNA replication without cell division?
Ans. DNA replication may occur without cells division like in the case of developing new mitochondria and chloroplasts. In cell division, the parent cell is divided into two daughter cells. But in case of repeated DNA replication without any cell division, then this DNA will continue to accumulate within the cell. This would increase the size of the cell nucleus, thus leading to cell expansion.
16. Analyse the events during every stage of cell cycle and notice how the following two parameters change
(i) Number of chromosomes (N) per cell
(ii) Amount of DNA content (C) per cell
Ans. (i) Number of chromosomes (N) in a cell
- G1 phase: Number of chromosomes is identical to that of the parent cell (diploid in body cells). There is a single chromatid per chromosome.
- S phase: DNA replication takes place, but the chromosome number is not altered. Each chromosome now has two sister chromatids, but the number of chromosomes (N) remains constant.
- G2 phase: The quantity of chromosomes is still the same, as the cell gets larger and ready for mitosis.
- M phase (mitosis): The number of chromosomes remains constant as the chromatids split up in anaphase so that the two daughter cells have the same set of chromosomes (N).
Therefore, during the cell cycle, the quantity of chromosomes (N) remains constant, but the organization of the chromosomes changes from a single chromatid in G1 to two sister chromatids in S and G2, before separating in mitosis.
(ii) The quantity of DNA content (C) per cell
- G1 phase: The DNA content is the baseline (C), one copy of the DNA per chromosome.
- S phase: There is DNA replication, and the content of the DNA doubles. Each chromosome now contains two sister chromatids, essentially doubling the content of the DNA (2C).
- G2 phase: The DNA content is still doubled (2C) as the cell gets ready to divide, and there is no additional DNA replication.
- M phase: When the chromatids are split into two daughter cells, the DNA content is reduced back to the baseline (C) in each of the daughter cells.
Therefore, DNA content doubles in S phase (2C) and subsequently is reduced half in the M phase (C) when the cell splits into two.
